The present invention relates to a metal clad insulating circuit breaker, and more particularly to an improvement of a solid insulating circuit breaker.
In known switching apparatus designed for a nominal voltage of more than 60 kV, gaseous insulating materials have been employed instead of solid insulating materials. For the necessary dielectric strength, the gas must be maintained at high pressure, and the gas pressure must be monitored periodically for detection of leakage and replenishment of gas lost. Solid insulating materials are free of this shortcoming, but the necessary heavy layers of solid material tend to crack due to the different rates of thermal expansion of the insulating material and of the metallic conductors embedded therein, and corona discharge and current leaks occur at the cracks. For this reason, solid insulating materials have been used only in electric machinery operating at maximum nominal voltages of 20 to 30 kV.
Recently, in large cities, with the growth in population and the excessive density of buildings and industrial institutions, the need for electric power has rapidly increased, but it is always very difficult to relocate transformer substations in the build-up areas of large cities.
Accordingly, the efforts of the art are directed at the reduction of the site and of the size of the transformer substation. Accordingly, an insulating breaker of small size is required; therefore, the insulating breaker has been manufactured in a small size, instead of using air insulation requiring the long insulation distance, by employing solid, liquid or gas insulation between the electrically conductive parts and the grounded member to shorten the insulation distance.
The breaker according to one of the above insulation methods, for example, the breaker of the 20-30 kV class with solid state insulation has been manufactured by locating inside the mold its live conductive parts such as lead wires, breaker unit and the like, injecting a molten electric insulating material composed mainly of epoxy resin into the space between the mold and the live conductive parts including the breaker unit, and forming all parts in one mold body.
In this case, even if the breakers have equal ratings, various mold types must be prepared according to the fitting conditions of the breakers (for example, the fitting dimensions of the breaker for the face of power panel). When the exterior form of the breaker or the using conditions so require the breaker must be manufactured each time by molding the breaker unit and the lead wires in one piece. Accordingly, various mold types are required and mold manufacturing become uneconomical.
In case of manufacturing of the circuit breaker for low voltage according to said molding method, as relatively large scale production of the breaker at one and the same electric rating is required, the adequate returns will result. But when the circuit for the breaker is of high or ultra-high voltage, the breaker according to the solid material insulation method is generally not required in large quantities, and as the molded produce becomes large and complex, the mold therefore becomes very costly and uneconomical to produce.
Conventional and prior solid insulating electric arrangements of this type are shown in FIGS. 1 and 2. Such arrangements are, generally, used for a breaker of the 20 30 kV class. In the arrangement shown in FIG. 1, a cylindrical live conductor portion 10 is enveloped by molded-on tubular members 12 and 14 of insulating material of uniform diameter. These tubular members 12 and 14 have, respectively, flanges 12a and 14a for fastening to each other in an air-tight relationship by employing a gasket 16, bolts 18 and nuts 20. Further, the surfaces of these tubular member 12 and 14 are shielded and grounded by a metallic conductor 22, so the insulation treatment corresponding to the insulating class is necessary in order to obtain the required dielectric strength. The dielectric strength is maintained by means of compressing the packing means such as rubber gasket 16. In this case, the compressive strength is imparted to the rubber gasket 16 by the fastening strength of bolts 18 and nuts 20, and, furthermore, the fastening strength of the bolts and nuts imparts the compressive strength Q to the rubber gasket 16 and the tubular members 12 and 14 which are composed of solid insulating material such as epoxy resin, and as a result the compressed rubber gasket 16 imparts the reactive force to tubular members 12 and 14. Accordingly, the flanges 12a and 12b receive the bending moment due to the above two forces P and Q, therefore the compressive strength required for the dielectric strength is restricted from the structural viewpoint. It is, therefore, uneconomical to manufacture the electrical arrangement of nominal voltage of more than 60 kV based on the principle of FIG. 1 because the molded-on tubular member 12, 14 requires a large increase of the mechanical strength to be applied thereon.
To achieve this, in the prior arrangement, spring members 24 is provided with the bolts 18 as shown in FIG. 2. However, in this case, the diameter R between the bolts arranged on the circumference of flanges 12a, 14a becomes large, and thus the bending moment increases, owing to the active and reactive forces P, Q.
It is, therefore, pointed out that the breakers using the solid state insulation are operable with only 20 to 30 kV capacities and are thus incompatible with systems in excess of 60 kV. This is due to the difficulty in molding the solid state insulation to a sufficient and precisely controlled thickness and due to fissures produced in the solid insulation owing to the difference between the coefficients of thermal expansion of the solid insulation and metallic parts which are embedded in the solid insulation.
It is, therefore, an object of the present invention to provide a circuit breaker of the metal clad type which overcomes the above drawbacks, namely, a circuit breaker arrangement in which the bending moment in the molded casing is reduced.
Another object of the present invention is to provide an electrical device of the metal clad type capable of using ultra-high voltage equipment, economically.
A further object of the present invention is to provide a relatively small-sized and highly reliable circuit breaker which can be manufactured inexpensively and easily.